Just thought I'd point it out, at no point was the number 5 mentioned at all...
I'm betting it'll be a remastering of the Xbox/Xbox 360 Halo games for Xbox One, with a port of Halo 4's multiplayer, it makes more sense really.
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The way i took it they just wanted to throw halo in because it sells, but didn't put a "5" there because they don't want to commit to anything yet in terms of naming.